Oil refineries may be faced with a serious problem of corrosion when they are used to treat certain “acidic” crudes. These acidic crudes essentially comprise naphthenic acids which are the cause of this very specific corrosion phenomenon since it occurs in a liquid medium which is a nonconductor of electrical current. These naphthenic acids correspond to saturated cyclic hydrocarbons carrying one or more carboxyl groups. The acidity of a petroleum crude oil is described by a standardized measurement according to Standard ASTM D 664-01. It is expressed in mg of potassium hydroxide necessary to neutralize 1 g of oil and is referred to as TAN (Total Acid Number). It is known in this technical field that a crude oil having a TAN of greater than 0.2 is described as acidic and can result in damage in the plants of a refinery.
This corrosion reaction is highly dependent on the local conditions, such as, for example, the temperature and the metallic nature of the wall in the plant concerned, the space velocity of the hydrocarbon and the presence of a gas-liquid interface. Thus, even after considerable research on the subject, refiners encounter great difficulties in predicting the scale of the corrosion reactions and their location.
One of the industrial solutions to this corrosion problem consists in using installations made of stainless steels, i.e. alloys of iron with in particular chromium and molybdenum. However, this solution is not employed to any great extent due to the high capital cost. Furthermore, this choice preferably has to be considered during the design of the refinery as stainless steels exhibit inferior mechanical properties to those of the carbon steels which are normally used and require an appropriate infrastructure.
The existence of these technical difficulties in the treatment of acidic crudes thus has the consequence that these crudes are generally sold to refiners at a lower price level than that of standard crudes.
Another solution to the problem of the treatment of an acidic crude oil, used by refiners in practice, consists in diluting it with another nonacidic petroleum crude oil so as to obtain a low mean acidity, for example of less than the TAN threshold of 0.2. In this case, the concentration of naphthenic acid becomes sufficiently low to produce acceptable rates of corrosion. However, this solution remains of limited scope. This is because some acidic crudes exhibit TAN values of greater than 2, which places an upper limit on their use at at most 10% of the total volume of crudes entering the refinery. Moreover, some of these mixtures of crudes with acidic crude sometimes result in the opposite effect desired, that is to say in an acceleration in the reactions for corrosion by naphthenic acids.
Another approach for combating this corrosion problem is the introduction into the acidic crude oil to be treated of chemical additives which inhibit or prevent attack on the metal wall of the plant concerned. This route is often very economical in comparison with that consisting in using the special steels or alloys indicated above.
Laboratory studies, such as that of Turnbull (Corrosion—November 1998, in Corrosion, volume 54, No. 11, page 922), have envisaged the addition of small amounts (of the order of 0.1%) of hydrogen sulfide to the crude oil to reduce corrosion by naphthenic acids. However, this solution cannot be applied in a refinery as hydrogen sulfide, which is a gas at ambient temperature, is highly toxic, which renders the consequences of a leak extremely serious and limits the use thereof. Furthermore, at a higher temperature, hydrogen sulfide itself becomes highly corrosive and will result, in other parts of the refinery, in a worsening of generalized corrosion.
U.S. Pat. No. 5,182,013 discloses, in order to solve this same corrosion problem, the use of other sulfur compounds, namely polysulfides comprising alkyl radicals of 6 to 30 carbon atoms.
More recently, the use of corrosion inhibitors based on sulfur and on phosphorus has also been disclosed.
Thus, patent EP 742 277 discloses the inhibiting effect of a combination of a trialkyl phosphate and of an organic polysulfide. U.S. Pat. No. 5,552,085 recommends the use of thiophosphorus compounds, such as organothiophosphates or organothiophosphites. Patent AU 693 975 discloses, as inhibitor, a mixture of trialkyl phosphate and of phosphoric esters of sulfurized phenol neutralized with calcium hydroxide.
However, organophosphorus compounds are very problematic to handle due to their high toxicity. Furthermore, they are poisons for the hydrotreating catalysts installed for purifying the hydrocarbon fractions resulting from the atmospheric and vacuum distillations. For these two reasons at least, their use in the field of refining is not desirable.